Answer to Question 1
The law of diminishing marginal utility states that each additional unit of a good consumed provides less and less additional satisfaction. Therefore, the amount that a person will be willing to pay for a product will fall as more of the product is consumed. This is one reason why demand curves slope downward.
Answer to Question 2
Price discrimination is the practice of selling different units of a good or service for different prices. For example, airlines' customers pay different prices for the same trip, pizzerias charge a lower price for the second pizza bought, or Microsoft charges a lower than regular price when it sells its Office software to students.